Vacuum fluorescent display (VFD) is commonly utilized in the application of a small display to provide a better contrast of brightness. VFD has the advantages of better brightness, wider visual angle, larger operating temperature range and lower production cost.
In VFD, the driving method for the filaments depends on the type of the filaments. Generally, there are two kinds of the driving methods for the filaments. One is the AC power driving method and the other is the dc power driving method.
For generating an AC power to drive the filaments of a VFD operated in a dc power supply, an AC sine wave signal is generated by using a transformer based on the LC oscillation effect resulted from an inductor and a capacitor. The frequency can be adjusted by changing the inductance of the inductor and the capacitance of the capacitor, the amplitude can be adjusted by changing the turn ratio of the coils of the transformer, and the phase can be adjusted by changing the central tap of the transformer and incorporating a Zener diode.
Although the circuit topology of the above traditional AC power control device is simple, it lacks of a feedback control loop. When the input voltage is changed, the output voltage will be influenced undesirably.